The invention relates to a method of multi-color printing wherein the vibrations in register deviations caused in the printed image by the feeding devices are determined by means of analysis of printed sheets and a control device.
In order to insure a high quality of printing it is known to inspect the entire surface of a printed material by means of an image pickup device. The image signals obtained with the image pickup device are processed in a control device. With such conventional means it is possible to improve the printing quality to a degree that register deviations caused by interferences can be remedied by means of the adjusting elements of the machine, and whereby the remaining register deviations are reduced.
The present invention makes it possible to further improve the printing quality without adding further devices to the machine.
The solution to the problem is found, according to the invention, in that from the image signals transmitted to the control device, by means of a defined number of printed sheets, the rhythm, i.e. frequency, of the register deviations in the printed image is defined, and from which preferably the period or frequency and the magnitude of the rhythmic register deviations are determined. The rhythm of the register deviations can be used in several ways to improve the print quality as follows: In a first approach the rhythm of the register deviations is used to determine, by means of the control device, those elements of the sheet feeding devices that cause the vibrations. By identifying these elements, they can be re-designed in regard to their structural form so that the vibrations are reduced. In cases wherein the sheet feeding device, in regard to its mechanical construction, e.g. in regard to its stiffness or oscillating nature, is arranged to be adjustable, it may be possible to adjust its mechanical features so that the rhythmic register deviations are almost eliminated.
A further possibility to take advantage of the information embedded in the constant rhythmic oscillations of the printing machine is found therein that this information can be used to produce the printing form within or outside the printing machine. In this case, the positions of the image points on the printing form are changed so that the rhythmic register deviations are reduced.
The rhythm of the register deviations can be determined by means of an image pickup device, by means of which the color play in defined printed image areas, preferably gray areas, are detected, or by analysis of register marks, or image elements that may be similar to register marks. The color play appears as rhythmic color variations, the size of which can be determined by means of color location oscillations.
The register deviations can also be related to color deviations for the purpose of establishing reasonable tolerances.
It is accordingly an object of the invention to provide a method for multicolor printing, wherein a printing material is transported through a printing machine by means of a cyclically operating feeding arrangement, the method which includes:
printing successively on at least one surface of the printing material a printed image;
generating with an image pickup device an electric image of each printed image from at least one side of the printing material;
transmitting the electric image of a plurality of printed images to a control device, and processing with the control device the electric images to obtain information relating to rhythmic vibrations in the machine causing register deviations.
According to the invention there is further provided a method for multicolor printing which includes determining from the information the period and magnitude of the rhythmic vibrations.
In accordance with another feature of the invention, the method includes determining from the period and magnitude of the rhythmic vibrations by means of the control device elements in the cyclically operating feeding arrangement that cause the vibrations.
In accordance with an added feature of the invention, the method includes adjusting the device elements causing rhythmic vibrations, so as to minimize the vibrations.
In accordance with an additional feature of the invention, the method includes determining with the image pickup device gray areas in the image, and the color play in the gray areas to determine register deviations.
In accordance with still another feature of the invention, the method for multicolor printing includes examining in the gray areas with the image pickup device multicolor horizontally and vertically oriented line or line-looking image elements for determining rhythmic register deviations.
The method for multicolor printing may additionally include determining periodicity in the multicolor horizontally and vertically oriented lines, and correlating the periodicity with the vibrations to determine machine elements causing the vibrations.
The method for multicolor printing according to the invention may further include the step of applying the rhythmic vibrations from the printing machine to the composition of the printing form in printing machines arranged for direct printing.
In accordance with a concomitant feature of the invention, there is provided the step of applying the rhythmic vibrations from the printing machine to a printing form produced outside the machine.
With the above and other objects in view there is also provided a multi-color printing method, which comprises the following steps:
printing a plurality of print substrates in a printing machine having a plurality of printing units;
scanning the print substrates;
determining a color register between individual ones of said printing units on a plurality of the print substrates and determining whether the color register shows a sheet-synchronous periodicity; and
if the color register shows a sheet-synchronous periodicity, determining a source of the periodicity and eliminating or minimizing a vibration causing the periodicity.
In accordance with another feature of the invention, the vibrations may be eliminated by superimposing a compensation variation of equal amplitude and out of phase by 180xc2x0.
In accordance with again a further feature of the invention, if the color register shows an asynchronous periodicity, examining all machine elements outside the printing units for correlation with the asynchronous periodicity, and eliminating or minimizing the source of the vibration.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for multi-color printing, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.